In wireless radio frequency (RF) communications and radar applications, it is often desirable to provide power amplifiers having high power over a wide frequency range. Transistor power amplifiers operating at high output power over wide instantaneous bandwidths typically tradeoff efficiency (RF power output divided by DC power input) for acceptable power output and gain performance across a predetermined frequency range. This tradeoff is required because the optimum load impedance for the output transistor is not presented to the transistor at each frequency within the operating range. Instead, a load impedance (also referred to as a broadband match) is presented to the output transistor to give acceptable, but less than optimum, performance across the entire intended frequency range.
Known RF systems have operated with less than desired performance in power amplifier efficiency across the operating frequency range. In these systems, the thermal capacity of the power amplifier is designed to handle the waste heat resulting from the amplifier inefficiencies. Some systems operating over relatively narrow bandwidths use impedance matching techniques to improve power amplifier efficiency at high output powers.
Increasing power amplifier efficiency over wide bandwidths has been improved using microelectromechanical systems (MEMS) RF switches to select matching components. An example of using MEMS RF switches to tune a power amplifier having relatively low output power over a range of frequencies is described in U.S. Pat. No. 6,232,841, entitled “Integrated Tunable High Efficiency power Amplifier,” issued May 15, 2001. Additionally, many power amplifiers are utilized in frequency hopping systems (i.e., systems in which the carrier frequency changes on a periodic basis). Frequency hopping occurs at relatively high speeds, requiring that the MEMS switches that select the proper impedance matching components be capable of operating reliably for extended periods at these relatively high switching speeds.
For applications requiring relatively high power output (e.g., desired output power of about fifty watts) and a wide (30–400 MHz) frequency range, for example, power amplifiers for military communications systems, several design parameters must be resolved. For example, the relatively large number of MEMS RF switches and matching elements needed to provide a very wide frequency range and the resulting physical space requirements which conflict with good RF design practices at high frequencies must be addressed. The proper placement of MEMS RF switches and matching elements in the output matching circuitry where impedance levels are very low due to the high output power level must also be addressed. Furthermore, these components must survive high current levels (due to high RF power at low impedance levels) and must have a very high Q so as not to degrade the optimum efficiency and negate the advantage of tuning the power amplifier. In a frequency hopping communications system, the number of cycles experienced by a MEMS RF switch should not exceed the MEMS switch life cycle limits, where a different matching solution is selected for each subsequent hop requiring a state change for several of the MEMS RF switches. If the hop rate is fast enough (as in a military jam-resistant communications system), the MEMS RF switches will be cycled many times per second. Typical mission profiles indicate that each of the MEMS RF switches could experience 1012 cycles. The accumulated number of cycles can be several orders of magnitude greater than the life cycle rating of a MEMS RF switch which has a typical life cycle limit of approximately 109 cycles. Furthermore, a configuration including a single point of failure, where a single MEMS RF switch failure severely impacts the performance of the power amplifier over the entire frequency range, must be avoided.
The importance of providing a matched load to the output transistor for optimum efficiency has been recognized in the past by those of ordinary skill in the art of power amplifier design, but implementing an actual matching circuit approaching a matched load over a wide frequency range at high output power levels has been relatively difficult to achieve. It would, therefore, be desirable to provide a matching circuit for reliably achieving high efficiency and high output power over a relatively wide frequency range. It would be further desirable to present the proper impedance to an output transistor across the entire desired frequency range, with reduced switching of additional matching elements, to locate MEMS RF switches and associated capacitors and inductors used for optimization in the circuit where these components are minimally stressed, and to prevent a catastrophic failure of the RF power amplifier system when a MEMS RF switch failure occurs.